The present invention relates to a method of processing edible frying oils and more particularly toward a close-coupled process for improving the odor and thermal stability of unrefined, unbleached soybean oil.
Soybean oil represents a readily available and relatively inexpensive source of nutritious vegetable oil. However, finished soybean oil processed by conventional industrial practice has the characteristic of readily evolving objectionable odors at frying temperatures, that is, temperatures within the range of about 350 degrees F. to about 400 degrees F. or higher. This odor has been variously described as a beany, grassy and/or fishy smell, and is quite pungent, markedly unpleasant and very unappetizing. In the case of finished soybean oil (by "finished" is meant refined, bleached and deodorized soybean oil), this objectional odor can occur in a matter of seconds upon heating at or near frying temperature.
A variety of processes have been proposed to eliminate the objectionable frying odor of soybean oil and some of these processes have been used commercially for many years. One such process calls for partially hydrogenating the soybean oil. See "Advances in Research on the Flavor Stability of Edible Soybean Oil" by J. C. Cowan, published in Food Technology, Vol. 19, No. 9, pp. 107-146 (1413-1452), 1965. Partial hydrogenation has been quite successful in increasing flavor stability of soybean oil for room temperature use or storage. However, the hydrogenation process must be carefully controlled to prevent the resulting hydrogenated soybean oil from obtaining a lard-like consistency. Hydrogenation has a high energy requirement and necessitates large storage facilities during the intermediate stages of the process. It also increases the price of the finished soybean oil since it produces stearin which must be removed by a winterization process prior to marketing. This winterization process increases costs, including additional energy consumption, and reduces the over-all yield of finished marketable oil.
A second method which is commercially used today for increasing the stability of soybean oil is deodorization (see the above cited J. C. Cowan article). This involves a vacuum steam stripping of the oil, optionally in the presence of citric acid, to improve the storage flavor stability of soybean oil. However, this stability is only for a short period of time and the process has little, if any, effect on the odor stability of soybean oil at frying temperatures. Accordingly, deodorization processes have only limited effectiveness.
Another method of increasing the flavor stability of soybean oil is described in U.S. Pat. No. 2,349,381, issued May 23, 1944 to Harvey D. Royce. The method described in this patent comprises heating the soybean oil at a temperature between 240 degrees C. and 300 degrees C. out of contact with air and in the presence of finely divided particles of zinc, magnesium, or tin for a period of between 10 and 120 minutes. This method, which has not been commercially successful, increases the flavor stability of soybean oil for room temperature use, such as for salad oil. However, it has little effect in eliminating the undesirable odors that develop when frying with soybean oil.
U.S. Pat. No. 3,780,076, issued Dec. 18, 1973 to Basil Papahronis and Walter Gibble and U.S. Pat. No. 3,758,532, issued Sept. 11, 1973 to Walter Gibble, describe processes for stablizing the odor and aroma of unhydrogenated refined soybean oil at frying temperatures by means of a process including the steps of deaerating the oil; saturating the oil with carbon dioxide; treating the saturated oil with a copper-chromium catalyst at a temperature of about 250 degrees C. for about one hour, optionally in the presence of carbon; filtering the treated oil; and then bleaching and deodorizing the oil. The finished, treated oil exhibits satisfactory performance at frying temperature. However, these processes have not been considered economical and have not been put to commercial use due to the high energy requirement during the treatment step and the cost of the catalyst which is expended in the filtration after each treatment step. Large storage tanks are required between various steps of the treatment process, increasing the amount of energy needed and decreasing the physical space available for the treatment process itself.
Another method of producing a flavor and aroma stable oil is described in U.S. Pat. No. 3,857,866, issued Dec. 31, 1974 to Walter Gibble and Edward Reid. This method comprises degassing refined unhydrogenated soybean oil to remove a substantial portion of atmospheric oxygen from the oil, dissolving carbon dioxide in the degassed oil and then dispersing a minor amount of finely divided, activated metallic salts and oxides including bleaching earths, clays, etc., and heating the dispersion in a carbon dioxide atmosphere to a temperature between 212 degrees F. and 260 degrees F. for a predetermined length of time. The treated oil is then filtered and deodorized to provide a finished oil having satisfactory performance at frying temperature. Like the previous treatment processes, this method is susceptible to contamination of the soybean oil during processing by exposure to oxygen in the air or by settling during storage between intermediate steps of the treatment. Additional processing steps are necessary to reduce the contaminants in the soybean oil.
Thus, there exists a need for a method of treating unrefined, unbleached soybean oil to produce a stable edible frying oil which reduces energy costs, eliminates the necessity of storage during intermediate steps of the treatment and reduces contamination of the soybean oil